SEOUL (Reuters) - South Korean President Moon Jae-in described Japan's wartime use of "comfort women" as a "crime against humanity" on Thursday in some of his strongest comments yet, sparking an immediate protest from his key ally in containing North Korea.

Moon said during a speech marking a national holiday commemorating Korean resistance to Japanese occupation - his first since taking office last year - that Japan was in no position to declare the emotionally charged issue settled.

Suga, speaking at a regular briefing, also urged cooperation between South Korea and Japan to tackle the threat posed by North Korea.

The two Koreas have pursued a thaw in relations that began ahead of last month's Winter Olympics in South Korea, but Seoul remains a key part of the international push to increase pressure on Pyongyang to abandon its nuclear weapons and ballistic missile programs.

Japan and South Korea share a bitter history that includes Japan’s 1910-45 colonization of the peninsula and the use of "comfort women", Japan's euphemism for women - many of them Korean - forced to work in its wartime brothels.

Japan apologized to the women and provided a 1 billion yen ($9.4 million) fund to help them under a 2015 deal with Moon's conservative predecessor, but South Korea has recently sought to revisit the issue.

"These details were agreed by South Korea and Japan and we find it unacceptable and extremely regrettable," Suga said.

HISTORIC TENSIONS

Moon, speaking at the site of a former jail where Korean independence fighters were imprisoned by Japanese forces, said South Korea was not looking for "special treatment" from Tokyo.

However, he hoped Japan pursued "sincere self-reflection" and "squarely face the truth of history and justice with the universal conscience of humanity".

Japan also formally complained on Monday after South Korea's foreign minister raised the issue at the top U.N. rights body, warning that it should not be allowed to harm bilateral relations at a critical time in East Asia.

A South Korean panel set up to investigate the deal concluded late last year that the agreement failed to meet the needs of the thousands of girls and women forced to work in Japan’s military brothels.

Over the objections of some in his conservative base, Japanese Prime Minister Shinzo Abe went on to visit South Korea during last month's Winter Olympics, but he restated Tokyo’s opposition to revising the 2015 agreement, telling Moon the deal was a promise between nations and the basis of two-way ties.

On Thursday Moon said South Korea considered Japan one of its closest neighbors and hoped to be able to move forward together.

"I hope Japan will be able to genuinely reconcile with its neighbors on which it inflicted suffering and will walk the path of peaceful coexistence and prosperity together," he said.

(Additional reporting by Kaori Kaneko and Tim Killy in TOKYO, and Yuna Park in SEOUL; Editing by Paul Tait and Michael Perry)